Modern radio communications system (e.g. WCDMA, Wideband Code Division Multiple Access) utilize digital modulation schemes in which both the amplitude and the phase of a complex base band signal are modulated onto the envelope of a Radio Frequency (RF) carrier, thus constituting a band pass RF signal. One way of a achieving such a band pass RF signal is to use Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM) of a switched-mode power amplifier operating at the carrier frequency.
In a switched-mode power amplifier, the power transistors are either in a fully conducting ON-state or in a completely non-conducting OFF-state, and a switched-mode power amplifier may achieve a much higher power efficiency (ideally up to 100%) than a linear power amplifier.
In conventional PWM (Pulse Width Modulation), the amplitude of a signal is mapped onto the width of a pulse at each sample, and a single pulse is transmitted by the modulator for each incoming sample. Further, in conventional PPM (Pulse Position Modulation), the phase information of the signal is mapped onto the position of the pulse, and PPM (Pulse Position Modulation) may be used together with PWM (Pulse Width Modulation) in a PWM/PPM in order to create a pulse sequence representing both the amplitude and the phase of the signal. FIG. 2 illustrates how the amplitude of a signal 1 sampled at Ts0 is mapped onto the width, i.e. duration, of a modulated pulse 3, and the phase of the signal sample is mapped onto the position of the pulse 3 within the time interval between two samples Ts=Ts1−Ts0, i.e. the sampling interval or sampling period.
Although the use of switched-mode RF amplifiers is still limited due to the high switching frequencies needed, the generation of the necessary band pass RF signal using PWM/PPM only requires switching at the carrier frequency and not a multiple of this frequency, as in the case of e.g. band pass Delta-Sigma modulation or low pass PWM. Thereby, the necessary digital and analogue circuitry for a PWM/PPM may be implemented as an integrated circuit, e.g. in the form of an RF ASIC (Radio Frequency Application Specific Integrated Circuit).
FIG. 1 is a block diagram illustrating a conventional conceptual switched-mode architecture, consisting of any suitable switch-modulator 2 arranged to modulate a base band input signal to present a pulse-sequence 3 forming a binary-level signal with correct switching positions to a power amplifier 4. Thereafter, the amplified pulse sequence 5 is filtered by a properly designed filter 6 tuned around the carrier frequency in order to filter out a correct amplified radio frequency output signal 7.
The technique explained in FIG. 2 is applied in the conventional arrangement illustrated in FIG. 3, in which the Cartesian coordinates of a base band signal I+jQ, i.e. the I-signal and the Q-signal, are converted into Polar coordinates by the converter 10. The amplitude-signal, A, and the phase-signal, φ, representing the Polar coordinates are modulated by a conventional, combined PWM/PPM 8, by which the amplitude-signal is mapped on the width of a pulse and the phase-signal is mapped on the position of said pulse within the sampling period of the baseband signal. Since the mapping of the input amplitude onto a pulse width is a non-linear function, i.e. a sine-function, an inverse (i.e. arcsine) pre-distorter is needed to obtain a linear output, and this correction is pre-calculated in the correcting calculator 11 in the illustrated arrangement. FIG. 4 further indicates the pulse sequence 3 created by the combined PWM/PPM 8 representing both the amplitude and phase of the base band signal, as described above. Thereafter, the pulse sequence 3 is amplified by the power amplifier 4, and the amplified pulse sequence 5 is filtered by the band pass filter 6, resulting in an amplified base band signal 7 on the output.
Related art within the technical field is disclosed e.g. in US2004/0246060, which describes a modulator for generating a two-level signal suitable for amplification by a switching mode power amplifier, such as a Class D amplifier.
However, the above-described conventional arrangements and related art involve several drawbacks. For example, a combined pulse width—and pulse position-modulation with a fixed sample period, Ts, may lead to “wrap-around”, or phase jump, of a pulse when the phase mapping extends over the +/−180 degree border. This “wrap-around” is illustrated in FIG. 4, in which the pulse representing the sample at Ts0 is “wrapped” within the sample period, since this first pulse cannot extend over to the next sample interval. Instead, a second pulse will be transmitted during the next interval, and this second pulse will represent the amplitude and phase of the second sample, at Ts1. This phase jump may also lead to missing or wrong pulse widths at the phase jump position.
It is further known within this technical field to combine the above-described PWM (pulse-width modulation) and PPM (pulse-position modulation) with band pass Delta-Sigma (DS) modulation. However, band pass DSM involves some drawbacks, such as a high out-of-band noise and a very high switching frequency. Normally, the sampling frequency fs=4·carrier frequency, such as in an fs/4 band pass DS-modulator.
A drawback with the combined PW/PP-modulation is the time granularity of a digitally defined pulse width and position, which restricts the achievable dynamic range due to quantization noise. At least 512 levels would be required for the width or positioning in order to reach 60-70 dB dynamic range, and this requires a clock frequency and a speed of the digital circuitry that is not achievable today.
Thus, it still presents a problem to achieve a switch-modulator architecture for a switched-mode radio-frequency power amplifier enabling linear amplification of radio-frequency signals over a large bandwidth with a high dynamic range and without “wrap-around” problems, suitable for implementation as an integrated circuit.